support HTML definition lists (<dl>, <dt>, and <dd>)

[chrispy-snps]

Fixes #172.

New convert_dt() and convert_dd() functions are added that follow the PHP Markdown Extra syntax:

https://michelf.ca/projects/php-markdown/extra/#def-list

If additional definition list dialects are requested in the future, a configuration option can be added to select the format.

No convert_dl() function is added; the child-tag conversion functions do all the work.

The regression tests are updated to test various structures. I also used Pandoc to confirm that all Markdownify results are converted back to the expected HTML source.

Note: This pull request requires that #171 be merged first; otherwise the test_dl unit test will fail.

Limitations

There are two limitations in this support, both related to the fact that blank lines are added outside the convert_dt() and convert_dd() function scopes.

Limitation 1 - multiple terms sharing the same definition are not handled properly (the term lines are separated by a blank line instead of kept directly adjacent):

<dl>
  <dt>term 1a</dt>
  <dt>term 1b</dt>
  <dd>definition</dd>
</dl>

Limitation 2 - a blank line is always inserted before definitions, causing them to signify paragraph-based definitions even when they were not:

<dl>
  <dt>term 1</dt>
  <dd>bare definition</dd>
  <dt>term 2</dt>
  <dd><p>definition in paragraph</p></dd>
</dl>